1. Field of the Invention
The present invention relates to a subscriber circuit, and more specifically to a subscriber circuit having a function for monitoring operation of a subscriber instrument such as a telephone set.
2. Description of Related Art
A subscriber circuit includes a monitor circuit for monitoring various operations of a subscriber instrument such as a telephone set. Referring to FIG. 1, there is shown a circuit diagram of a typical example of a hook monitor circuit, which is one of monitor circuits and which first operates in the subscriber circuit when a subscriber makes a telephone call.
The shown hook monitor circuit is generally designated with Reference Numeral 1A, and connected to a pair of subscriber lines "Tip" and "Ring" which are interconnected within a telephone set when a handset is lifted (off hook). The shown hook monitor circuit includes a resistor R.sub.11 connected at its end to the subscriber line "Tip" and another resistor R.sub.12 connected at its end to the subscriber line "Ring", an amplifier A.sub.I having an output connected to the other end of the resistor R.sub.11, an amplifier A.sub.2 having an output connected to the other end of the resistor R.sub.12, and a power feeding controlling circuit 11 for detecting a level on each of the subscriber line "Tip" and the subscriber line "Ring", so as to supply a predetermined level of power supply voltage to the subscriber telephone set through the amplifiers A.sub.1 and A.sub.2, the resistors R.sub.11 and R.sub.12 and the subscriber lines "Tip" and "Ring".
The shown hook monitor circuit also includes an amplifier A.sub.3 for detecting and amplifying a voltage across the resistor R.sub.11, an amplifier A.sub.4 for detecting and amplifying a voltage across the resistor R.sub.12, an adder 12 for summing output voltages of the amplifiers A.sub.3 and A.sub.4, and a comparator 13A for comparing an output voltage of the adder 12 with a reference voltage Vr.sub.11 and for generating an active hook monitor signal HS when the output voltage of the adder 12 becomes higher than the reference voltage Vr.sub.11.
The amplifiers A.sub.1 to A.sub.4 and the power feeding controlling circuit 11 operate with a power supply voltage Vbb of -48 V, which is the same level as that of a voltage supply for supplying an electric power to the telephone set. However, the comparator 13A operates with a power supply voltage Vcc of +5 V, which is the same as an operating power supply voltage for signal processing and controlling circuits (called "internal circuits" hereinafter) within the telephone set. Accordingly, the power supply voltage Vcc is different from the power supply voltage Vbb.
In the above mentioned hook monitor circuit 1A, when the handset of the subscriber telephone set is hooked off or lifted off, the subscriber lines "Tip" and "Ring" are interconnected within the telephone set, so that there is formed a closed loop composed of the power feeding controlling circuit 11.fwdarw.the amplifier A.sub.1 .fwdarw.the resistor R.sub.11 .fwdarw.the subscriber line "Tip".fwdarw.the subscriber line "Ring".fwdarw.the resistor R.sub.12 .fwdarw.the amplifier A.sub.2 .fwdarw.the power feeding controlling circuit 11, and a loop current flows through the closed loop thus formed. This loop current causes a voltage drop across each of the resistors R.sub.11 and R.sub.12, and the voltage drop is detected and amplified by each of the amplifiers A.sub.3 and A.sub.4. The two amplified voltage drops are summed by the adder 12, and then, supplied to the comparator 13A. In this comparator 13A, the output voltage of the adder 12 is compared with the reference voltage Vr.sub.11 predetermined by a voltage source E.sub.11.
The comparator 13A generates an active hook monitor signal HS (high level) when the output voltage of the adder 12 becomes higher than the reference voltage Vr.sub.11 and an inactive hook monitor signal HS (low level) when the output voltage of the adder 12 becomes lower than the reference voltage Vr.sub.11. In the off-hook condition, since the voltage drop across the resistors R.sub.11 and R.sub.12 are large because of the loop current, the comparator 13A ordinarily generates the active hook monitor signal HS of the high level.
In the conventional subscriber circuit as mentioned above, if the power supply voltage Vbb (ordinarily, -48 V) supplied to the amplifiers A.sub.1 to A.sub.4 and the power feeding controlling circuit 11 of the hook monitor circuit 1A as well as the other subscriber instruments drops to a level (for example, -30 V) which is out of a normal operation voltage range of these amplifiers and power feeding controlling circuit, the amplifiers A.sub.1 to A.sub.4 and the power feeding controlling circuit 11 no longer normally operate, and an error hook signal having an abnormal level is often generated.